In developing bacterial speck (Pseudomonas syringae pv. tomato) resistant cultivars the reaction of tomato plants to fenthion (Lebaycid 50EC) can be used as an indicator in differentiation between resistant and susceptible genotypes. The presence of concentric light brown necrotic lesions on tomato leaves after spraying with Lebaycid 50EC corresponds with resistance to this disease. The method appeared to be simple, reliable and useful in tomato breeding program. Genetic analysis in F1, F2 and backcross segregation populations generated from crosses cv. Ontario 7710 (sensitive) and A100 (insensitive) indicated that sensitivity to fenthion in tomato is governed by one completely dominant gene Fen.
